Impacts of free nitrous acid on stabilizing food waste and sewage sludge for anaerobic digestion

[Display omitted] •FNA pretreatment can significantly improve the solubilization of organic waste.•pH change within the system can adversely impact the FNA pretreatment performance.•Effective FNA concentration may decrease due to NO2– consumption by denitrifiers.•FNA-pretreated organic waste does not always show a higher biogas production in AD.•FNA at 1.0–5.2 mg FNA-N/L effectively controlled H2S-odor in the organic wastes. This work investigated the effectiveness of free nitrous acid (FNA) in enhancing organic waste solubilization to improve biogas production in anaerobic digestion (AD). The results indicated that FNA pretreatment can enhance soluble organic content and control H2S odor in tested organic wastes, including food waste, sewage sludge, and their combination. However, a significant decrease (>50 %) in FNA concentration was found in the reactors, possibly due to denitrifier-driven NO2– consumption. Biochemical methane potential (BMP) tests showed a 25 ± 8 % enhancement in CH4 production in the reactors fed with mixed substrate pretreated with 2.9 mg FNA-N/L. However, the presence of NO2– (325.6–2368.0 mg N/L) in some BMP reactors, due to carryover from FNA pretreatment, adversely affected CH4 production (>55 %) and prolonged lag time (>4.2 times). These findings are valuable for researchers and practitioners in waste management, offering insights for implementing FNA pretreatment to enhance the biodegradability of organic wastes in AD.